Compact camera modules have become a standard component in mobile devices such as mobile phones, tablets, hand held game computers, and note books. A camera module consists of PCB board, an imaging sensor module and a lens module. The lens module consists of a lens assembly and a housing shielding it from unwanted light and environmental influences. The housing may also be shared with the housing of the complete camera module. The outer contours of a compact camera module are in many cases designed as drop-in component into mobile devices.
The mass volume requirements together with the increasing drive for reducing costs triggered the development of wafer level based methods for producing and packaging the camera modules and related image sensor and optics.
The aim of packaging is to integrate the several optical, mechanical, environmental and electronic functions of a compact camera module and a lens module. The functional elements consist of a CMOS or CCD image capturing device, the imaging lenses together with optical functions such as IR filters, AR coats and light blocking structures such as baffles etc. In most cases, micro lenses and color filters are positioned on the image sensor surface.
The lens elements are usually formed by injection moulding or glass pressing. Integrated lens stacks relating to lens assemblies based on wafer level manufacturing have been disclosed in WO2004027880. In this process, lens elements, spacers and other optical functions are manufactured at the wafer level. After singulation (i.e. separation of the wafer into individual modules) integrated lens modules are obtained.
Wafer level manufacturing of opto-electronic components in general assumes a wafer to wafer assembly of the optics wafer with the image sensor wafer. The aim is to reduce costs through maximizing the simultaneous processing of components followed by a singulation, usually dicing, step. This assumption is based on the very high yields using state-of-art manufacturing front end processes for electronic components on silicon wafers. These processes benefit from a decades track record of process development using silicon as a substrate. However, the processes for manufacturing optical components on wafer level are based on different materials (glass, polymer) and processes (injection moulding, UV, thermal replication, glass pressing). In addition, refractive optical structures require extreme, i.e. high, shape accuracies with comparably high aspect ratios. So, in many cases, the yield involving manufacturing of optics on wafer level is lower than may be obtainable for electronic components. As a result an image sensor module wafer with good yield may be assembled on an optical wafer with a lower yield. Reject optics may therefore be combined with good image sensors, resulting in reject compact camera module despite the presence of good electronic components. A feature of a wafer to wafer approach is the possible or inherent footprint mismatch between the optics and the image sensor resulting in increasing manufacturing costs for the optics and resulting compact camera module.
Another feature involves the control of the Back Focal length (BFL) of the imaging optics. The control of the BFL within microns is a main contributor to the yield in manufacturing compact camera module. The BFL is to a large extent determined through the wafer level control on of the thickness and shape tolerances in all optical and spacer layers of the integrated lens stack.
The wafer level module technology mainly adopts wafer level fabrication technique in electronic products to miniaturize volumes of electronic products and reduce fabrication costs. The wafer level module technology is also applied in the fabrication of wafer level optical lens modules, such that volumes of wafer level optical lens modules can be greatly reduced comparing to that of conventional lens modules. Wafer level optical lens modules are consequently utilized in camera modules of cellular phones, for example.
US2011222173 relates to a method of fabricating a wafer level optical lens substrate, comprising: providing a substrate; forming at least one through hole on the substrate and forming a flange on a side wall in each through hole; and forming a lens on the flange in each through hole and embedding the lens with the flange.
US2009022949 relates to a process for producing a functional-element-mounted module, comprising the steps of disposing a substrate having mounted thereon a functional element having a mounting portion and a resin sealing plate formed therein with an opening corresponding in position to the functional portion of the functional element as opposed to each other at a predetermined distance; and impregnating and filling a sealing resin between the substrate and the resin sealing plate utilizing a capillary phenomenon.
In addition, when assembling a lens module upon the image sensor module, the distance between the bottom optical surface of the lens module and the image plane has to be very accurately controlled. This can be performed through active alignment assembly methods, wherein the image is projected on an image sensor and the quality of the resulting focal position is measured. According to the result, the lens module is vertically displaced to a position where an optimal image quality is obtained. The whole procedure of measuring and adjusting is time consuming and requires expensive assembly with in line focus length measurement.
From U.S. Pat. No. 3,532,038 there is known an optical system in which a transparent base member is provided with lenticular lens cavities, which cavities are filled with a refractive fluid, the surface of which fluid is covered with a cover member. The cover member is provided with an aperture plate, on which finally a second base member is present, which is also provided with lenticular lens cavities, which cavities are likewise filled with a refractive fluid.
From US 2004/0100700 there is known a method of manufacturing a microlens array, wherein the cavities in a mould are filled with a UV curable resin, whilst the resin outside the cavities is removed by placing a transparent quartz board on top of the mould. The fluid present in the cavities is then formed into a plurality of separate lenses, whereupon a second UV curable resin layer is applied to the transparent board, which resin layer is cured by making use of the already formed separate lenses. The excess amount of the cured second resin layer is removed by using an organic solvent. Only one layer of replicated lenses is mentioned in said document, which lenses are separately arranged and do not exhibit any interconnection.
From International application WO 03/069740 in the name of the present inventor there is also known a replication process by which an optical element is formed.
WO2012100356 relates to a method for manufacturing a plurality of optical devices, comprising the steps of: providing a replication tool, the replication tool comprising a replication surface defining an array of replication cells, each replication cell comprising a lens replication portion and a spacer replication portion, bringing the replication tool and a support in contact with each other with replication material between the replication surface and the support; causing the replication material to harden, wherein, during the step of causing the replication material to harden, the lens replication sections are caused to be kept at a distance from the support.
From the above state of the art there are thus known methods by which optical systems are obtained which are made up of separately manufactured optical elements, as a result of which the dimensions of such systems may be considered to be large. In addition, the positional accuracy, viz. in the X, Y and Z directions (between the lens surfaces) of such systems may be called critical.
From the above state of the art there are thus known methods by which lenses are replicated on substrates to obtain a single sided our double sided lens wafer. These substrates may be provided with coatings and/or diaphragm/aperture structures. Spacer wafers are provided between the lens wafers in order to achieve the total optical track length of the system.
A feature of these substrates is that the thickness is too high, i.e. stiff substrates have a thickness typically above 300 microns, and such a thickness needs be eliminated. In addition, glass substrates are regarded as expensive materials, particular when tight thickness control, i.e. below 5 microns, comes in question. Furthermore, tight thickness control is also a cost issue for the spacer wafers. In addition, lens substrates and spacer wafers must be assembled with adhesives, resulting in strict controlling XY and Z positions. Furthermore, some transparent substrates have undesirable optical properties, such as refractive index, Abbe number, resulting in unwanted interference with the optical system.
And the method of replicating of lenses within the apertures of a spacer wafer has an effect on the height, but still requires a tight tolerance of the stiff substrate used. In addition, the control of the shape of the lenses is somewhat difficult, especially the risk of the formation of bubbles, and/or a multistage process is required, comprising steps of, inter alia, filling holes with polymer, curing thereof, flattening of filled spacer wafer and replicating lenses on the filled holes.
Thus it is an object of the present invention to provide a method for manufacturing an optical unit in which the desired dimensional precision of the lens system can be achieved without the dimensions of the optical unit increasing undesirably.
Another object of the present invention is to provide a lens assembly, in which well defined lens shapes are present.